Resonant torsion oscillators are known, however, they are not widely used in imaging devices such as laser printers. One problem with oscillators in scanners is that the scan efficiency of such a typical galvo device is substantially less than that of a rotating polygon mirror conventionally used for such laser scanning procedures. Other problems associated with torsion oscillators include bulk associated with the materials used for the springs, magnets and coils; frequency drift; and instability. In general, these problems and others have prevented or discouraged use of torsion oscillators in applications such as optical systems.
There is a need, therefore, for an improved galvo device having increased scanning efficiency. There is also a need for a scanning device having a scanning efficiency approaching or exceeding that of a rotating polygon mirror.
Oscillating devices may be etched or cut as a micro-electromechanical system (MEMS) device, preferably from a single piece of silicon. The MEMS device provides a small, relatively compact device that can be illuminated by a light source such as a laser beam to scan the resulting beam of light across an imaging element such as an electrophotographic drum in a laser printer. If operated at, or near, its resonant frequency, the MEMS device is a resonant galvanometric (galvo) device.
The resonant galvo device includes a central mirrored plate suspended by two extensions of the plate material. The plate material extensions are integral with a surrounding frame. Preferably the mirrored plate, extensions, and frame are cut or etched from a single silicon wafer. A coil of conductive wire or permanent magnets are placed on the plate to provide a first magnetic field, and a reflective surface is formed on the plate to create a mirror.
The entire resonant galvo device is located within a second magnetic field that opposes the first magnetic field provided by the coil or magnets on the plate. Like the first magnetic field, the second magnetic field may be provided by a coil or wire or permanent magnetic. At least one of the first or second magnetic fields is provided by current passing through a coil of wires to provide a variable magnetic field. The variable magnetic field exerts a force on the magnets which causes rotation or twisting of the plate about its extensions (torsion bars). The spring rate of the extensions and the mass of the plate provide a rotational spring-mass system with a specific resonant frequency. The galvo device functions as a laser scanner when a laser beam is directed at the oscillating surface of the mirrored plate.
In a preferred embodiment, the invention provides an improved optical scanning system. The optical scanning system includes a resonant oscillating device having a first magnetic field and a mirrored surface. Also included in the system is a first light source for directing a first beam of light to the mirrored surface of the resonant oscillating device to provide a first reflected scan beam. A second light source is provided for directing a second beam of light to the mirrored surface of the resonant oscillating device to provide a second reflected scan beam. The second reflected scan beam is offset a first distance from the first reflected scan beam. A second magnetic field in the system interacts with the first magnetic field to provide torque to the resonant oscillating device for scanning the first and second reflected scan beams across a surface to provide first and second scan lines on the surface substantially simultaneously as the resonant oscillating device oscillates under the influence of the first and second magnetic fields. The first and second scan lines are spaced apart from one another by a second distance.
The concept of a “light source” is broad. For example, a first and second light source may be implemented by providing two separate lasers that are directly modulated or a laser and a beam splitter with external modulation. The laser and the beam splitter produce two beams of light and therefore the combination constitutes both a first light source and a second light source in this case.
The invention is not limited to two light beams. For example, a VCSEL (Vertical Cavity Surface Emitting Laser), by its fabrication method, is easier to make into an array, and for example an eight laser array may be used to produce eight light beams that may each be used in the present invention.
In a preferred embodiment, the invention provides a method for increasing the throughput and efficiency of an optical scanning system. In the optical scanning system, a current is applied to a coil at a drive frequency to create an oscillating magnetic field (e.g. the first or second magnetic field) and to cause the resonant oscillating device to oscillate at resonant frequency as the resonant oscillating device is illuminated by the first and second light sources thereby providing the first and second reflected scan beams.
An advantage of the invention is that the apparatus and method enable increased throughput without the need for increasing the frequency of oscillation of the resonant oscillating device. For example in a printer where data is encoded onto the light beams, the addition of multiple sources does not change the total scan time, nor does it change the usable print time, relative to the speed of the oscillating device. However, it does change the usable print time relative to a data clock by N, where N is the number of sources. In other words, more print data can be transmitted in a given period of time and thus the throughput is increased.
Also, to achieve greater scan efficiency, one embodiment of the invention uses bi-directional scanning of light beams across a surface. Another advantage of the two beam embodiment is that increased scans per inch may be obtained for the same oscillating scan frequency as an oscillating scanner having only a single light source. This enables a two laser printer using the system to provide double the print resolution as compared to a single laser system, assuming both systems have the same oscillation speed. In the alternative, it enables the oscillating scan frequency to be cut in half for a two laser system and still achieve substantially the same print speed or throughput as a single laser system. Similarly, an eight laser system utilizing the present invention achieves eight times the throughput as compared to a single laser system.
In the two beam embodiment, a single beam may be monitored to determine the position and movement of both beams. For example, a single sensor may be disposed in the path of the first reflected scan beam and the sensor output may be used as information relating to the position of both beams. Thus, two beams are monitored for the price of one sensor.